The Brown Headed cowbird (Molothrus ater) is a parasitic bird of North America. They used to follow the bison around and consume the insects associated with them. With the expansion of cattle in North American, the brown-headed cowbird populations have also expanded which has raised some interesting questions. Female cowbirds lay their eggs in the nests of other birds and allow those birds to raise their young. This type of parasitism is called brood parasitism. Because the cowbird is typically larger than these birds their young die in the process of raising the cowbird. Because the cowbird was not tied to a nest this allowed them to follow the bison which was a food rich source. It is unknown if they evolve this strategy to follow bison or were able to follow bison because of this strategy. They also are able to produce more eggs in a season than a typical bird, up to 3 dozen. Bird populations of the birds they parasitize have suffered as a result of this and efforts have been made to try to reduce this. Using feed designed for small birds, not spreading seed on the ground, and avoid feeding sunflower seeds, cracked corn, and millet will reduce the number of cowbirds in your area. It is important to note that the birds are covered by the Migratory Bird Act so it is illegal to remove their eggs from a nest or harm them in any other way without a permit. In some states, if they threaten an endangered species you can obtain a permit to trap the birds.
Female cowbirds watch female birds of another species and determine where their nest is located. While the other female is away the cowbird will sneak into the nest, damage or remove one or more of the eggs, and lay her egg. The cowbird may also eat the egg instead of damaging it or removing it from the nest. The cowbird eggs have a short incubation period and typically hatch 1st. They also grow larger than the other nestlings which means they can more effectively beg food from the parents, leaving the other nestlings to starve. They often can be larger than the parents before they fledge which they do after 10 to 11 days. They have been documented parasitize over 220 species of birds but females will typically only target one species of bird. Some species like the Yellow Warbler (Setophaga petechia), recognize cowbirds eggs and will reject them or build another nest on top of them. There is evidence that species that do not reject the eggs do so because it is energetically costly to abandon the whole nest. Cowbirds are also known to Kirtland’s Warbler (Setophaga kirtlandii) and Black-capped Vireo (Vireo atricapilla) are rare birds that have been impacted by the cowbird. It is hard to quantify how much other species are impacted by forest fragmentation also has negative impacts in populations and is associated with an increase in cowbird populations.
Grasslands are the preferred habitat of cowbirds. The increasing amount of grasslands, particularly outside of the great plains, has also allowed cowbird populations to expand. They actually prefer human-modified habitat to their past habitats. They prefer edge habitat where they have access to both open fields and forest but they are found in all habitats besides forest interiors.They feed mostly on seeds and occasional will consume insects. Agricultural grains are a favorite food of the cowbird which is part of the reason they prefer human-modified habitats. In the winter time, they gather in mixed species flocks with other blackbirds. In Kentucky, one such flock has over 5 million birds. The oldest known cowbird was over 16 years old.